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Signs and Symptoms of a Bad Air Filter

Vehicles use combustion engines that rely on air in order to function properly. This air is combined with gasoline in order to generate heat energy, this results in the generation of horsepower.

Needless to say, the air filter is important. It prevents dirt, bugs, and other particulates from entering the engine. With that said, it also must allow adequate amounts of air to reach the engine. Over time it can accumulate enough particulates to clog the filter and affect the performance of the vehicle.

Let’s look at a number of common symptoms of a bad air filter.

Signs of a Faulty Air Filter

1. The Filter Looks Dirty

comparison of dirty and clean air filter

One of the most obvious bad air filter symptoms is that the filter looks dirty. If your car is brand new, the filter will be white or nearly white. However, it is bound to become dirty over time from air contaminants, which will make the filter look brown or black.

So if you suspect that your filter isn’t functioning properly, you should first give it a visual inspection. Fortunately, that is very easy to do — all we need is some good lighting. If the filter is covered in dust, dirt, and debris, you should replace it.

2. The Check Engine Light is On

The Check Engine light (CEL) can turn on for a number of reasons. One of those reasons might be deposists building up in the engine. Car engines, can take in thousands of gallons of air for a single gallon of fuel that they burn in the combustion cycle.

If this happens, the best thing to do is to take your car in for servicing. A mechanic will check your air filter, as well as other components and determine the cause of the problem. They can also check the service codes and reasons for the CEL coming on.

3. Poor Fuel Economy

changing air filter

In a combustion engine, there needs to be enough oxygen in order to produce enough power. When the air filter is blocked, this prevents an adequate air/fuel mixture from being achieved. In order to compensate for the lower levels of oxygen, the engine will consume more fuel in order to maintain horsepower.

On the other hand, if your car is newer, then it should have a fuel-injected engine with a computer that calculates the amount of air that goes into the engine. A dirty air filter shouldn’t affect fuel economy in a significant way.

4. Misfiring Engine

If you are having a hard time starting your car, or you have to rev the engine in order for it to start, the reason could be a low air-to-fuel ratio. When there is too much fuel in the ratio and not enough air, that can result in the engine flooding and the spark plugs becoming polluted. In turn, that can cause your car to misfire or have trouble starting.

A reluctant start or a rough idle are typical warning signs of a bad air filter.

5. Horsepower Reduction

Lack of air can also impair your vehicle’s performance, making accelerating sluggish. If you notice that your car responds slower than usual or it jerks while accelerating, it probably isn’t getting enough air. According to a study by, engines with a dirty air filter accelerated 6-11% slower than vehicles with clean air filters.

6. Unusual Noises from the Engine

checking air filter

When your car is in park and the engine idling, the engine should produce a smooth humming sound. However, if you are experiencing a rattly idle, hear hiccups or coughing, or feel unusual vibrations, this is usually caused by dirty or damaged spark plugs.

As you might expect, the spark plugs become dirty and damaged due to a clogged air filter. Changing the air filter and the spark plugs will bring idling engine back to normal.

7. Black Smoke or Flames from the Exhaust Pipe

When air can’t reach the engine, it fails to burn off fuel during the combustion cycle. As a result, we may notice fuel leaking through the system to the exhaust pipe. In addition, you may hear popping sounds or even see a flame at the end of your exhaust.

Not only does this issue waste fuel, but it can be dangerous to both your vehicle and the environment. It’s best to have this issue checked as soon as possible.

Richard Reed
I've been a General Manager of a moving company and I've also been a Professional Mover for over 30 years. I've driven flat beds, reefers, dropdecks, moving vans, heavy machinery, etc. In my time as a Mover I've driven over 1,000,000 safe miles. My days of moving and driving truck are past me but The Vehicle Lab allows me to share the knowledge I've gained over the past 40 years.

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